A VVVF-inverter-based method for precisely controlling an AC motor without using a speed detector has been proposed in recent years (see Patent Documents 1 and 2, for example).
In such a method for controlling an AC motor without using a speed detector, a speed estimate is computed based on, for example, a detected current value to control the VVVF inverter. However, when the computed speed estimate is not correct due to current detector failure, wiring breakage, or other defects, overcurrent frequently occurs if any of the defects is left unaddressed, possibly resulting in breakage of parts of the VVVF inverter. It is therefore necessary to quickly sense such an abnormal speed estimate.
Patent Document 1: Japanese Patent No. 3,732,784 (FIG. 6, Equation 28)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-64905 (FIG. 1)